This invention relates to an telephone apparatus in which an external channel is capable of being selected and acquired.
When a terminating request signal is received in a conventional exchange system in which an ISDN circuit is accommodated as a line wire, even though the circuit which has received the terminating request signal may be in a state reserved for acquisition, processing is executed in exactly the same way as when the circuit is not in the acquired state.
By way of example, in a case where an ISDN circuit is acquired and a transmitting operation is performed in an exchange system accommodating the ISDN as a line wire, first the user reserves one of the information channels B1, B2 of the ISDN circuit and then enters dialing information. When the entry of all dialing information ends, the main apparatus sends a transmit-request signal, which has designated a reserved information channel, and executes transmission processing.
In a case where a terminating request signal has been received from the ISDN circuit, a response is made depending upon the actual state of use of the B channel designated by the terminating request signal received.
However, if, in the example of the prior art described above, a terminating request signal which has designated the B1 channel is detected from the ISDN circuit while the user is in the course of entering dialing information from an extension telephone upon having reserved the B1 channel of the ISDN circuit, the B1 channel will not yet be in a reserved state. Consequently, a response will be made to the terminating request without any notification whatsoever being given to the user. As a result, the user, who has reserved the B1 channel and performed the transmitting operation, is likely to become confused because a transmission cannot actually be made despite the fact that the proper operation has been carried out.
In other words, even though the user believes that he has acquired the circuit, a response is made to the terminating request signal. Therefore, even if the person who has reserved acquisition of the circuit subsequently performs a transmission operation, another individual will be using the reserved circuit and it will be impossible to make the transmission. This can cause the user to become confused.
For example, if, when the acquired line wire is the B1 channel from among the three channels (the two B channels and the D channel) of the ISDN circuit, a terminating request signal which has designated the B1 channel is received and a response is made to this signal, a transmission can no longer be made over the acquired line wire at the terminal connected to the extension. This can cause great confusion in terms of operation.